Comparing the adsorption mechanism of Cd by rice straw pristine and KOH-modified biochar

Biochar was considered as an effective and novel sorbent for cadmium (Cd) adsorption from aqueous solution. In this study, the adsorption isotherm investigations were conducted to examine the effect of biochar produced via pyrolysis from rice straw on removing aqueous Cd before and after modification by 2 M KOH solution. Langmuir and Freundlich adsorption isotherms can preferably describe the adsorption process. Results showed that the highest adsorption capacity of pristine rice straw biochar was 12.17 mg g^-1. The chemically modified rice straw biochar showed greater Cd adsorption capacity of 41.9 mg g^-1, which was more than three times that of pristine biochar. Increase of surface area and changes of porous structure, especially the functional groups on the surface of modified biochar, were the major contributors to its more efficient adsorption of Cd. The possible mechanisms for Cd adsorption by biochar mainly involve (1) surface precipitation by forming insoluble Cd compounds in alkaline condition and (2) ion exchange for Cd with exchangeable cations in the biochar, such as calcium ions, which were confirmed by FTIR, XRD, SEM, and zeta potential determination.